1. Field of the Invention
The present invention relates to a process for preparing substantially spherical polymer particles, in particular polymer particles of substantially uniform particle size.
2. Description of Prior Art
Polymer particles of a substantially uniform particle size, in particular particles of an inorganic polymer such as amorphous silica, are used for various applications, e.g., as carriers for chromatography, such as high performance liquid chromatography. Chromatographic separation methods have become increasingly important in recent years, not only for the purpose of chemical analysis, but also in industrial (preparative) separation processes, e.g., in the pharmaceutical industry as outlined in, e.g., Process Engineering, February 1984, pp. 26-31.
The polymer particle material used for such chromatographic separations should meet various requirements which are mentioned, e.g., in J. Chromatography 83, 1973, pp. 5-9. For instance the material should be composed of small spherical particles of a uniform particle size. With a narrow particle size distribution, a smaller flow resistance in the chromatographic column is obtained, and the smaller the particle size, the greater the separation efficiency. However, a lower limit of about 5 .mu.m in particle size is determined by the difficulty in packing the chromatography columns with any degree of reproducibility.
Another important parameter is the pore structure which depends on the type of chromatography to be employed. The desired pore diameter also depends on the substances to be separated. In the above-mentioned article from J. Chromatography a distinction is made between macropores which should be ten times smaller than the particle diameter, and micropores which should be about two and a half times the diameter of the molecule of the substance which is to be absorbed in the chromatography separation process. The pore volume should be as large as possible; however, with increasing pore volume, the stability with regard to the pressure used in packing the column (up to 500 bars) decreases and it has been found that the pore volume should not exceed about 1.7 cm.sup.3 /g.
In the methods of preparing such particles known to the present inventor, a large particle size distribution is obtained so that it is necessary to fractionate the resulting product which consequently increases the production costs.